1. Field of the Invention
The present invention relates to an optical characteristic measuring apparatus and a measuring method using the optical characteristic measuring apparatus, and more particularly to a technique of measuring an optical characteristic such as the reflectance of a thin film with a higher precision.
2. Description of the Background Art
A microspectroscope is known as a typical optical characteristic measuring apparatus for measuring optical characteristics (optical constants) such as the reflectance, refractive index, extinction coefficient, and film thickness of a thin film by applying light to the thin film formed on a substrate for example and spectroscopically measuring the light reflected therefrom.
A conventional microspectroscope is configured for example as disclosed in FIG. 1 of Japanese Patent Laying-Open No. 11-230829. The microspectroscope includes an illuminating optical system directing an illuminating light emitted from a light source through a half mirror to a sample to be measured that is set on a table, and an converging optical system bringing the light reflected from the sample to be measured to a diffraction grating and to a monitoring-purpose optical system. The diffraction grating functions as spectroscopic means for splitting an observation light from a measurement region on the sample to be measured, and converges the spectrum on a line sensor. From the spectrum measured with the line sensor, an optical characteristic is calculated. The monitoring-purpose optical system uses a relay lens to form an enlarged image of the sample to be measured, on a two-dimensional CCD camera. The enlarged image of the sample to be measured that is produced by the CCD camera is used for checking the position of measurement and for rough focusing.
As described above, the conventional microspectroscope uses the illuminating light emitted from the light source for both of the purpose of measuring the spectrum and the purpose of focusing.
Usually, in the case where an optical characteristic is to be measured, it is necessary to measure the spectrum in the wavelength range for which the optical characteristic is to be measured. In order to allow a user to adjust, with the eyes, the focus on a sample to be measured, the observation light should include a wavelength range in the visible band. Therefore, it is necessary to employ a light source of the illuminating light that has a relatively wide wavelength band including the wavelength range for what is to be measured as well as the visible band. At the same time, for the focusing purpose, a certain level of brightness as well as a relatively wide observation view should be ensured. Thus, it is also necessary that the illuminating light has a relatively large beam diameter.
Here, in order to measure optical characteristics with a higher precision, it is necessary to use an illuminating light that has a less intensity variation and is stable. However, it is difficult to directly obtain an illuminating light having a wide wavelength band, being stable and having a relatively large beam diameter. Accordingly, a generally employed configuration uses a light source generating an illuminating light of a relatively small beam diameter and an expander expanding the beam diameter of the illuminating light. An expanded beam diameter, however, results in a decreased light intensity (quantity of light) per unit area of the illuminating light.
In other words, while a smaller beam diameter is preferred in terms of further enhancing the precision in measurement of optical characteristics, a larger beam diameter is preferred in terms of focusing. As seen from the above, the conventional microspectroscope potentially involves the requirements that cannot be satisfied concurrently, and the improvement of precision in measurement of optical characteristics has been limited.